To achieve good image quality for both dark and bright scenes, image processing systems implement a variety of brightness adjustment procedures, commonly referred to as auto-exposure (AE). In typical image processing systems a number of factors can be controlled to affect the brightness (luminance) of the image. The most frequently controlled factors comprise variable optical integration time, variable gain settings of the amplifiers and variable reference voltages of the analog-to-digital converters in the system. In some systems, programmable digital gains are also available.
Many current systems provide for fast convergence to a desired or target brightness, but also greatly increase signal noise in the image, which results in poor image quality.
Given the number of controllable factors and their associated discrete increments of adjustment, there is a need for a method that would not only converge to the preset luminance level of the image, but would also optimize signal-to-noise ratio, provide an efficient speed of adjustment and be stable enough not to create frequent brightness adjustments in response to small or short changes in the scene illumination. Another important goal of an efficient AE method is that it should avoid “flicker” resulting from the oscillating light intensity of fluorescent illumination.